Determining strain-induced crystallization of natural rubber composites by combined thermography and stress-strain measurements

Strain induced crystallization is essential to the physicochemical properties of polymer materials, but is difficult to investigate, as it usually requires X-ray sources in combination with stretching machines. We improve and validate a recently developed method which allows the calculation of the crystallinity index using easily available thermography and stress-strain data. For natural rubber, the method is shown to be reproducible and delivers results quantitatively comparable to spectroscopic methods such as wide angle X-ray scattering. The incorporation of different amounts of carbon black is shown to increase the level of crystallization and to change the shape of the strain-crystallization curves. Additionally, crystallinity during partial retraction is investigated and reveals that crystallization characteristics change at sufficiently high strain.